More than 5% of the people in industrialized nations have significant hearing problems that range in severity from modest difficulty with speech comprehension to profound deafness. Hearing loss is age-related, as about 4% of people under 45 years old and about 34% of those over 65 years old have debilitating hearing loss. In most cases, the cause is related to degeneration and death of hair cells and their associated spiral ganglion neurons.
The ear is composed of four main sections: the external ear, middle ear, inner ear, and the transmission pathway to the hearing center in the brain. The inner ear is a capsule of very dense bone containing a fluid that communicates with the middle ear. Small bones within the middle ear (the malleus, incus, and stapes) transmit sound energy from the tympanic membrane to the oval window at the entrance to the cochlea of the inner ear. The action of the stapes at the oval window exerts pressure on the fluid within the cochlea. The pressure is transmitted through the cochlea, ultimately causing a second window, the round window to oscillate. A basilar membrane that defines the fluid-filled chambers of the cochlea then transmits the oscillations to the organ of Corti, which contains about 13,000 mechanosensory cells called hair cells. Hair cells are located in the epithelial lining of the inner ear (in the cochlear organ of Corti, as mentioned), as well as in the vestibular sensory epithelia of the saccular macula, the utricular macula, and the cristae of the three semicircular canals of the labyrinth. The cochlear hair cells send signals to the cochlear spiral ganglion, and the clustered neuronal cell bodies convey those signals to the cochlear nucleus of the brain stem (see FIGS. 5A, 5B, and 5C).